The technology of this invention is classified in both Class 423 and Class 55. The applicable subclasses in Class 423 are believed to be Subclasses 239 and 244.
In U.S. Pat. No. 4,259,304, issued to Steiner, coal is treated in a sequential manner to provide an absorbent for SO.sub.2. The coal is partially oxidized with sulfur dioxide and then activated by contact with superheated steam followed by impregnation with vanadium. The coal is used to remove SO.sub.2 from an SO.sub.2 -containing gas.
Another desulfurization agent is disclosed in Hagiwara et al, U.S. Pat. No. 4,540,553, wherein a hardened product is prepared to act as a desulfurization agent. This hardened product comprises cement which is mixed with limestone or dolomite and is hardened by incorporating water. The process of the patentees utilizes this hardened product in a fluidized bed system to remove SO.sub.2 from an SO.sub.2 -containing stream.
Many catalysts have been developed for the select reduction of nitrogen oxides contained in exhaust gases. Usually these nitrogen oxides are reduced in the presence of ammonia. U.S. Pat. No. 4,048,112 (Matsushita et al) discloses a catalyst for the selective reduction of nitrogen oxides contained in exhaust gases in the presence of ammonia. The catalyst comprises vanadium oxide on a anatase form of TiO.sub.2. The vanadium is believed to act as a catalyst to accelerate the reaction of NO.sub.x with ammonia to provide water and nitrogen.
In 1980, U.S. Pat. No. 4,186,109 issued to Atsukawa et al, disclosed a catalyst for the selective reduction of nitrogen oxides prepared by mixing a silicic acid material with a lime material to form a slurry of calcium silicate crystals which is then formed onto a carrier. This catalyst is made to function as a NO.sub.x -reduction catalyst in the presence of SO.sub.x. It is recognized that SO.sub.x -containing combustion gases act to deactivate and choke NO.sub.x decomposition catalysts. While the patentees recognize that titania is a viable carrier for these decomposition catalysts, the cost of titania is extremely high. The patentees' invention provides a lightweight, inexpensive catalyst which is resistant to poisonous constituents such as SO.sub.x. The carrier for the particular crystalline calcium silicate may be zirconia, vanadia or titania.
In 1988, at least three patents issued for catalysts for the reductive conversion of nitrogen oxides contained in waste gases. The first of these is U.S. Pat. No. 4,742,037, Baiker et al, which discloses impregnation of a support with a vanadium alkoxide. The resultant catalyst can be used in the presence of sulfur oxides and at temperatures as low as 150.degree. to 350.degree. C. A division of the last patent was issued as U.S. Pat. No. 4,789,533. The latter patent claims the process for converting the nitrogen oxides, while the former patent claims the catalyst for the conversion. Finally, U.S. Pat. No. 4,789,531 to Eichholtz et al discloses a NO.sub.x -removal system comprised of an active coke/active carbon and a reducing agent such as ammonia, carbon dioxide or hydrogen. The adsorbent is precharged with the reducing agent to provide for a better system for the removal of nitrogen oxides.
A dry flue gas desulfurization process described by S. Kudo et al was presented at the First Combined Flue Gas Desulfurization and Dry SO.sub.2 Control Symposium on Oct. 25-28, 1988 in St. Louis, Mo. In addition to this disclosure, an article was written in Combustion Science and Technology by Merryman el al for the reduction of NO in the presence of fly ash. This article can be found at volume 20, pages 161-163.
In Berg, U.S. Pat. No. 4,806,319, a process is disclosed which simultaneously reduces sulfur and nitrogen oxides by contacting those oxides with petroleum pitch. When SO.sub.2 and NO.sub.2 are mixed with inert gases and contacted with residual fuel oil, asphalt or pitch, the SO.sub.2 and NO.sub.2 are reduced to compounds which will remain in the petroleum distillate. The inert gases exit from the hydrocarbonaceous material.
In U.S. Pat. No. 4,400,363, a method is described for the removal of sulfur oxides and nitrogen oxides from exhaust gases in a traveling bed which first removes the sulfur oxides and then subsequently, in the presence of ammonia, reduces the nitrogen oxides. This invention requires that the sulfur oxides be removed to have an effective NO.sub.x reduction system in the second part of the exhaust ga reaction.
Hamada et al received U.S. Pat. No. 4,469,662 relating to a process for removing both sulfur and nitrogen oxides in a dry process by passing a waste gas stream through a bed of carbonaceous adsorbent. The sulfur oxides are removed mainly by the carbonaceous adsorbent. The invention is based upon the discovery that carbonaceous adsorbents have a small capacity for adsorbing ammonia, but that carbonaceous adsorbents holding sulfuric acid have an increased capacity for ammonia. Thus, the system is a self-supplied system for removal of both nitrogen and sulfur oxides in a dry process. The process may be conducted from room temperature up to 200.degree. C.
As demonstrated in Atsukawa et al U.S. Pat. No.4,009,244, lime or limestone slurries containing alkali metals or alkaline earth metals have also been used to remove NO.sub.x and SO.sub.x from exhaust gases.